Capacitors are employed in digital and analog devices for a variety of purposes, including sample and hold circuits, data converters, filters, and circuits for storing electrical charge, blocking DC voltage levels, and stabilizing power supplies. Typical capacitors used in semiconductor devices are of various types, such as a metal oxide semiconductor (MOS) type, a polysilicon-insulator-polysilicon (PIP) type, a metal-insulator-metal (MIM) type, etc., wherein the type of capacitor employed typically depends on the application (e.g., analog or digital) and desired response characteristics of the device.
In many analog circuit applications, variations in the capacitance value of the circuit capacitors is undesirable. Such capacitance variation is troublesome, for example, in sample and hold circuits and data converters, wherein the operational performance of the entire circuit depends on stable capacitance values. Capacitance values may vary with device temperature and/or applied voltage, where the capacitance changes are believed to be caused by a variety of physical properties of the circuit capacitors. For instance, PIP capacitors suffer from capacitance variations believed to be caused by the doping characteristics of the polysilicon capacitor electrode plates, and as such, these devices exhibit fairly large changes in the capacitance as a function of applied voltage.
Voltage dependent capacitance variation is sometimes expressed or quantified in terms of a voltage coefficient of capacitance (VCC), typically measured in parts per million per volt (ppm/V) for a first order coefficient Vcc1 and in parts per million per volt2 for a second order coefficient Vcc2. In the design and fabrication of high precision analog circuitry, it is desirable to provide capacitors having small VCC values. MOS type capacitors may also suffer from parasitic effects, particularly where the capacitor is located proximate the substrate. MIM type capacitors may be advantageously fabricated in upper interconnect layers of a semiconductor device wafer to mitigate such parasitic effects. MIM capacitors are further desirable, since the electrode plates are fabricated from conductive metal materials, whereby the polysilicon doping issues associated with PIP capacitors are avoided.
Another impediment to fabrication of high precision analog circuits is dielectric absorption in device capacitors, also known as dielectric relaxation, hysteresis, soakage, etc. Dielectric absorption involves small amounts of excess charge being absorbed or released by a capacitor dielectric material after the capacitor has been charged or discharged. If the voltage across a charged capacitor is brought to zero (e.g., shorted) for a short time, the capacitor will appear to “self recharge” slightly after the discharge circuit is opened. Dielectric absorption is believed to affect all capacitors to differing degrees, wherein the amount of dielectric absorption for a particular capacitor depends primarily on the type of dielectric material used and the amount of dielectric material in the capacitor. Dielectric absorption may be quantified as the percent of charge trapped or stored in a capacitor dielectric (as opposed to the charge stored on the capacitor plates) that cannot be removed quickly. This percentage may be approximated as the ratio of the equilibrium value “self recharge” voltage to the voltage before discharge, and is typically expressed in parts per million (ppm).
Capacitance variations in high precision analog circuits are undesirable, and may lead to unacceptable device performance. Accordingly, there is a need for fabrication methods for creating semiconductor devices having capacitors with low dielectric absorption and VCC values.